Terribly fat Australian upstart lieutenant culpable? (2,5)
I believe the answer is:
at fault
'culpable?' is the definition.
(I've seen this in another clue)
'terribly fat australian upstart lieutenant' is the wordplay.
'terribly' is an anagram indicator.
'australian' becomes 'A' (common abbreviation).
'upstart' becomes 'u' (start of the word 'up').
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'fat' with letters rearranged gives 'atf'.
'atf'+'a'+'u'+'lt'='AT FAULT'
(Other definitions for at fault that I've seen before include "To blame, culpable" , "To blame, responsible for mistake" , "In the wrong" , "Guilty" , "Rightly taking the blame" .)